Repairs to under-slab ductwork

[tomkatohio]

I have an under-slab duct run of about 20 feet with two perpendicular branches of about 5 feet each. The house is about 30 years old and I know that the galvanized ducts have leaked groundwater during a few periods of heavy rain. I am considering alternatives to repairing and/or replacing the ducts. The area has a cathedral ceiling with no attic, so I don't think an overhead supply would be an option. Also "relining" systems seem less than effective. Are there options such as running rigid PVC or flexible ducts through the existing duct work? Although I have been told that flex ducts reduce the airflow considerably.

I would rather break up a portion of the floor that the entire run. If all else fails I may need to consider other alternatives such as baseboards in the rooms furthest from the furnace.
Thanks for any opinions or sympathy you might have.
Tom in Ohio

[REP]

The only problem with running PVC is that in the sizes you would need it will be on the expensive side..If you reline with standard metal duct you will be in the same situation in a few years.I wouldn't even think of flex duct.You could use rigid duct board (yes it coimes in round sizes)but you would be reducing the airflow because the diameters would be smaller.
I assume you mean electric base board when you mention baseboard and that would heat your rooms nicely if sized right but elec heat tends to be expensive.
If you could afford the PVC that would be the best answer but you wouuld have to bust up the floor to do it.
The only other answer would be to install mini split HPs in the rooms affected,or the baseboard.
GOOD LUCK

[saylor240]

I have always liked the PVC idea....yes it will be expensive but last forever....you may have some noise issues, cause your reducing the size. i guess you could use flex duct, like the kind they use under trailer homes, but may get torn up while installing...this would be interesting dilema...good luck....

[genduct]

When the duct was first installed, the concrete was supposed to completely surround the duct so that even when the galvanized duct rotted, the hole in the concrete was still there. You probably have earth exposed in your duct hole.

So, dig it up ( pretty expensive) or go to hydronic baseboard for heat (probably as comfortable as low heat from your slab duct) and a little more comfortable than a ceiling register system. Your location is really important here since we don't know if you are heat or colling dominate area.

Finally the overhead duct is best done if you can create duct soffits below the existing ceiling and INSIDE THE BUILDING'S: AIR, THERMAL AND VAPOR BARRIER

[Shophound]

I would recommend against breaking up your slab for any reason. A slab's concrete and rebar are a monolithic unit when the concrete is poured all at once. By breaking up a slab, fixing ducting or plumbing, and then repairing the rebar and repouring concrete, it's no longer a monolithic unit due to "cold joints" where the old concrete joins the new. Sure, it's done all the time, but it compromises the slab.

I just had all of the cast iron sewer piping under my home's slab replaced. The plumbers tunneled underneath the slab to access it. We would not entertain any busting up of the slab to do it, nor did the plumber we ultimately chose for the job recommend it. For ducting in the slab gone bad I would opt to abandon it and seek alternatives. I would fill in every supply and return opening with expanding foam and then a layer of floor leveler to prevent moisture migration into the house through the abandoned ducting.

[skibme]

Soffits , Mini splits or a High velocity system.

[TwincamDave]

First let me ask, do you have down flow equipment? If so you will have to replace the system to up flow. How does the industrial look effect you?

[tomkatohio]

I really appreciate all of the comments and suggestions.

After these I'm thinking that having some ducts run to the ceiling might be the next option. I could get to each rooms through the attic.

I have an upflow furnace in northern Ohio.

Would releasing the warm air at ceiling height (14 feet over living room cathedral ceiling) result in a warm ceiling and a cold floor? Would a standard blower be strong enough to create sufficient flow at that height? I would guess that this might be more efficient for the cooling season though. I should probably consult a professional huh?

Thanks again,
Tom

[genduct]

I think you already know the answer to the question form the question you asked.
No easy ( aka cheap) solution. Baseboard convectors is going to be best for heat. Maybe coupled with an airhandler for AC that has a hot water coil for Spring and Fall mild weather. keeping the duct inside the building envelope and not in the vented attic is always the perfered

[luxuryheatingco]

Have you looked into restoring the ducts with a liner? I don't know if this is against the forum policies but being that I'm not trying to "plug" my own company (it's a contractor that we recommend for work that's outside our scope - nothing in it for us), you should visit www.restoreyourducts.com - they have a system that will coat your slab ductwork with a rubberized liner. If you're really looking to abandon the ductwork in the slab, the suggestion of a mini-split might be your best option.

[tomkatohio]

I wonder where the greatest heat loss would be. A well-insulated duct running between the ceiling joists in an un-heated attic, or the old ducts running under the slab. Don't have a cost yet on re-lining ducts.

[genduct]

that's a good observation/ question about the uninsulated duct in the slab?
Balanced the advantage of floor diffusers and the small amount of warming from the duct to the concrete, HMMMMM
Well cooling would be great given the earth temp but what about the 120 degree air with the heat? the only way to know is to see how much the temp drops from the discharge. I suspect that unless it is low and slow air then the diff won't be more than a few degrees and it could be aurgued that it is warming up the slab at least a little.

[acwizard]

I have seen the relining technique, it is quite effective as far as sealing up any holes. The R- value of the duct in the slab is probably very low. Now may be the time to upgrade your home's heating and cooling system. You should determine a budget and then explore what you could have installed for that price. There are numerous ways to go about it, some being more costly than others. You may want to consult an engineer or a good design build contractor.

[tedkidd]

I've never moddeled duct in slab. Wonder what those losses would be.

If you have an audit, ask if they can model that. I'd be disenclined to cut holes in the ceiling unless 2 part spray foam was part of workscope.

[ACFIXR]

What kind of slab do you have?

That makes a difference in your choices.

Here in Tucson we saw cut slabs all the time during remodeling. No big deal for monos or stems. Big deal for post tensions.

Find a remodeler and/or plumber that does alot of remodeling and they will know what is typical for your area. 2 day job at the most. No big deal just messy.

[sktn77a]

A third vote for at least researching the "relining" option. I've seen it done for main drains and see no reason why it shouldn't work well for in-slab HVAC ducting.

[tomkatohio]

Thanks again for the great comments - I appreciate the help you provided to an amateur and a complete idiot when it comes to HVAC systems.

I have split level with what probably is a 4" monolithic slab poured between footers. I'm pretty sure the leaking ducts were covered with fill sand. The furnace is in the lower level and the main duct I'm concerned about runs from the furnace about 20 feet directly under the slab under the living room with 1 terminating register and 2 perpendicular runs of about 8-10 feet. The duct looks like it is 8' X 14". My worry is not just the water leakage and rust, but potential mold growth in this duct.

I would love to upgrade my system, but my budget would probably not permit it. If relining would solve the water problem perhaps that is the best option. I should still consult a local company to see what options they come up with.

[tedkidd]

Mold requires moisture AND food source. Good air filtration should keep that risk down.

[lynn comstock]

A duct run through the attic will work if the pressure losses in that run are less than the other branch ducts pressure losses. This is a design and workmanship issue: Manual D duct design, duct sealing and R8 insulation, minimum.

To get the heated air at the ceiling level down to the floor, use a ceiling fan in the downflow mode. Your existing system, when not heating, allows room air to stratify. How good or bad has that been?

[TwincamDave]

"Would releasing the warm air at ceiling height (14 feet over living room cathedral ceiling) result in a warm ceiling and a cold floor?"

It will eventually "stack" and move to the floor. With that said the slab will be cooler than the ceiling. A/C will kick butt.....